The present invention relates to a spot welding machine, and more particularly to a machine for both series and indirect spot-welding and is particularly useful in the assembly/manufacture of railway vehicles, etc.
In recent years, in order to make train coaches lighter and improve their quality, the manufacture of vehicles made of stainless-steel and aluminum-alloy metals, instead of ordinary steel, has been on a rapid increase.
A railway vehicle comprises blocks which are fastened together, namely, an underframe (floor), sides, ends, and a roof. The basic structure of each block consists of a reinforced panel comprising a parallel-cross frame covered by outer plates. The assembly of railway vehicles, unlike an automobile assembly line, is characterized by the fact that the parts are large and that a wide variety of shapes and structures (combinations of different plate thicknesses) require welding. For this reason, the welding machines used in such assembly have raised a number of problems that have not been resolved in the prior art.
In particular, in the case of stainless-steel railway vehicles, spot welding is employed for the entire assembly of the blocks ranging from the frame to the plates in order to keep welding distortions to a minimum for better appearance as well as to prevent the corrosion resistance inherent in such material from deteriorating. The spot-welding in this case usually is direct spot welding. The quality and performance efficiency of the spot welding process will, therefore, have an influence on the quality and productivity of the vehicles assembled.
Direct spot welding is so designed that a workpiece is held between two electrodes fitted to a spot gun and welded after spots have been predetermined. Because of this, if an electrically non-conductive body (an insulator) or an obstacle like a reinforcement part is present on the back side of the workpiece, or if the material is excessively thick, direct spot welding cannot be put into practice. In such a case, therefore, ring or plug welding (arc welding) is performed. However, in performing such weldings, the need also arises for such associated processes as drilling, bead-grinding, and accessory or final plate addition which results in a reduction in work efficiency and a lower productivity.
On the other hand, if the assembly procedure is modified so that an obstacle is welded later so as to avoid the interference with the spot gun, the work procedure during welding is made harder, and joining the parts thus becomes difficult, leading to a decrease in quality and work efficiency.
If the cross section of a part is large, the size of the spot gun must be correspondingly large, which contributes to a decreased work efficiency resulting from an increase in equipment cost and power consumption and in a lower welding speed.
Under these circumstances, and irrespective of the condition of the welding site, a one side welding device has been required which invariably allows for spot welding from one (the outer) side only. In particular, for such large work as railway vehicles, and in view of specific situations where there is a wide range of combinations of thicknesses of plates involved, the development of a highly productive and high quality spot welding device has been hoped for.
It is a general object of the present invention to provide a spot welding device capable of both series and indirect functions that can be used without exception for spot welding from one side.